1. Field of the Invention
The present invention relates to improvements in an electric power system for an automotive vehicle and, more particularly, to an electric power system which has both a high rated voltage source and a low rated voltage source.
2. Description of the Prior Art
In recent years, automotive vehicles provide an electric load which operates at a high efficiency by using a high-voltage electric power. Japanese Patent Provisional Publication No. 1-185197 discloses an electric power system applicable to an automotive vehicle having such a high-voltage load. Such an electric power system is provided with a generator, which generates 48-volt direct-current electric power, a 48-volt battery and a DC--DC converter (direct-current to direct-current converter) which converts the 48-volt direct-current electric power to a 12-volt direct-current electric power. The generator is connected to the 48-volt battery and a high-voltage load, such as a blower motor. A low-voltage load, such as a head lamp or tail lamps, receives 12-volt electric power from the DC--DC converter.
However, with such a conventional electric power system, since the DC--DC converter always operates for the conversion from 48-volts to 12-volts even if the head lamps or tail lamps are not in use, an energy loss is caused by the DC--DC converter.